The possible causes of bone swelling and pain below the kneecap of a 12-year-old boy are: trauma, septic osteomyelitis, osteosarcoma, etc. The treatment varies, and is broadly categorized into conservative and surgical treatments. 1. Trauma: 12-year-old boys are prone to knee trauma due to their large amount of exercise. If the examination excludes fracture, it can be treated conservatively, such as braking, using some drugs such as Yunnan Baiyao aerosol. If the X-ray examination found that there is a fracture, it can be fixed with a plaster or surgical treatment. 2. Septic osteomyelitis: Septic osteomyelitis is a purulent infection of the bone and joints, mostly seen in children and adolescents. Once diagnosed, adequate amount of antibiotics should be used immediately for anti-infection treatment, commonly used drugs include amoxicillin, cephalosporin and so on. If the local symptoms are not relieved after 36 hours of antibiotic treatment, surgery should be carried out in time to avoid the development of chronic suppurative osteomyelitis. 3. Osteosarcoma: Osteosarcoma is a malignant tumor of bone tissue origin, the treatment plan is radical surgical resection combined with chemotherapy. If the tumor size is large and difficult to be removed, surgical resection can also be performed after neoadjuvant chemotherapy. It is recommended that a 12-year-old boy with pain and swelling of the bone below the knee go to the hospital as soon as possible to find out the cause of the disease in time and be treated accordingly. Do not delay to avoid aggravation of the condition.